The term Planet Nine is relatively recent in astronomy. Early astronomers speculated about a “Planet X” beyond Neptune to explain anomalies in the orbits of Uranus and Neptune (especially in the late 19th and early 20th centuries).
Earlier, the idea of a hidden planet beyond Neptune had circulated under the name “Planet X”, particularly in the late 19th and early 20th centuries when astronomers searched for anomalies in the orbits of Uranus and Neptune. Pluto, discovered in 1930, was initially thought to fill this role, but was later found to be far too small. After Pluto’s reclassification as a dwarf planet in 2006, the “ninth planet” slot was open for new hypotheses.